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Mallards were caught at a duck decoy located near Oud Alblas in the Alblasserwaard, The Netherlands, from March 2010 until February 2011. Each captured mallard was marked with a metal ring, sexed and aged. Cloacal and oropharyngeal samples were taken for detection of current LPAIV infection, and blood samples collected for detection of antibodies to AIV. The cycle threshold (Ct) value, which is the first real-time amplification cycle in which matrix gene amplification was detected, was used to assess the degree of viral shedding from the cloaca and oropharynx. The Ct-value is inversely proportional to the number of virus particles in a sample. Feather samples were taken to assess the origin of birds (resident, local migrant, distant migrant) using stable hydrogen isotope analysis. For more details see Appendix 2.

Abstract1. Similar to other infectious diseases, the prevalence of low pathogenic avian influenza viruses (LPAIV) has been seen to exhibit marked seasonal variation. However, mechanisms driving this variation in wild birds have yet to be tested. We investigated the validity of three previously suggested drivers for the seasonal dynamics in LPAIV infections in wild birds: (1) host density, (2) immunologically-naïve young, and (3) increased susceptibility in migrants. 2. To address these questions, we sampled a key LPAIV host species, the mallard Anas platyrhynchos, on a small spatial scale, comprehensively throughout a complete annual cycle, measuring both current and past infection (i.e. viral and seroprevalence respectively). 3. We demonstrate a minor peak in LPAIV prevalence in summer, a dominant peak in autumn, during which half of the sampled population was infected, and no infections in spring. Seroprevalence of antibodies to a conserved gene-segment of AIV peaked in winter and again in spring. 4. The summer peak of LPAIV prevalence coincided with the entrance of unfledged naïve young in the population. Moreover, juveniles were more likely to be infected, shed higher quantities of virus, and were less likely to have detectable antibodies to AIV than adult birds. The arrival of migratory birds, as identified by stable hydrogen isotope analysis, appeared to drive the autumn peak in LPAIV infection, with both temporal coincidence and higher infection prevalence in migrants. Remarkably, seroprevalence in migrants was substantially lower than viral prevalence throughout autumn migration, further indicating that each wave of migrants amplified local AIV circulation. Finally, while host abundance increased throughout autumn, it peaked in winter, showing no direct correspondence with either of the LPAIV infection peaks. 5. At an epidemiologically-relevant spatial scale, we provide strong evidence for the role of migratory birds as key drivers for seasonal epizootics of LPAIV, regardless of their role as vectors of these viruses. This study exemplifies the importance of understanding host demography and migratory behaviour when examining seasonal drivers of infection in wildlife populations.